Secretion of relaxin by the monkey has been studied during the menstrual cycle and during pregnancy. The role of the corpus luteum in relaxin secretion was investigated by determination of hormone levels in ovarian venous blood before and after luteectomy. Decreases of greater than (80 percent were observed within 30 minutes.) Administration of hCG to monkeys during the luteal phase of the nonfertile menstrual cycle resulted in 1) a prolonged luteal phase, 2) an increased peripheral progesterone level followed by decreased blood content of this hormone and 3) an increasing blood level of relaxin during hormone administration. Similar hCG administration during the early and middle follicular phase as well as during the periovulatory period did not enhance peripheral relaxin levels. These studies indicate a differentiated corpus luteum is required for relaxin secretion. Investigations of the effects of the hypothalamic releasing hormones on relaxin secretion may offer other approaches to investigation of the control and regulation of relaxin secretion. Other laboratory species should be investigated.